1. Field of Invention
The present invention relates to lightweight concrete products, and more particularly, to such products where zeolite is a major component; and in one aspect, to a zeolite-based binding material for use in hazardous waste stabilization.
2. Description of Prior Art
Conventional lightweight concrete products are made of Portland cement and/or lime, together with slag, pulverized fuel-ash, and other siliceous fine aggregates. Air or other gas (usually hydrogen from the reaction of aluminum powder in lime water) is introduced into a slurry composed of these materials to produce aerated or cellular concrete products. A uniform cellular structure is formed when the mixture sets. Autoclave curing is generally necessary if lightweight concrete products are to be produced with an acceptable level of strength and production yield.
Zeolite powder can be made by grinding either synthetic zeolite or natural zeolite minerals. Zeolite powder can be pre-treated depending on the application. A hydraulic cement is a dry powder which, upon mixing with water, sets and becomes a hardened solid mass forming a water-resistant product. Lime contains mainly calcium oxide or calcium hydroxide.
Zeolite is a porous silicoaluminate mineral. It has a very large surface area generally greater than 20 m.sup.2 /g. Its solid surface can be activated to have high surface energy after calcination at temperatures greater than 400.degree. C. When subsequently immersed in water, the surface generates a large amount of air and heat due to adsorption. This heat increases the temperature of the air in pores or adsorbed on the surface of zeolite particles. The expansion of the air volume results in foaming and volume expansion of fresh concrete during the mixing and pre-storage periods. In addition to its foaming function, zeolite can also react with calcareous materials to form calcium silicoaluminate hydrates which contribute to the strength of the concrete. Therefore zeolite has the potential for use as a basic raw material to produce aerated concrete.
The use of zeolite minerals in Portland cement to increase strength and decrease porosity has been described in PCT Int. Appl. WO 92 17,413, to R. S. Chase. The Chase patent application indicates that zeolite, as do other SiO.sub.2 -containing materials, has the pozzolanic activity required to contribute strength of Portland cement-based cementitious binders. No claims for other uses of zeolite (e.g., its air-generating capabilities) different from those of conventional pozzolanic materials such as fly ash, slag and silica fume were made. A similar patent was issued in U.S.S.R (see SU 1,738,774, to G. I. Ovcharenko et al., 1992). The Ovcharenko patent describes a composition of blended cement containing 15-30wt. % zeolite, 40-60 wt. % gypsum and 18-40 wt. % Portland cement.
The use of zeolite in production of lightweight aerated building materials has been reported in JP 03 93,661 to S. Kureha et al, 1991. According to that disclosure, zeolite was utilized in relatively small amounts, e.g., 20 wt. % zeolite, 80 wt. % cement and water/solid ratio 0.7. Zeolite was calcined at 500.degree. C. and cooled at -20.degree. C. The aerated concrete had a bulk density of 420 kg/m.sup.3 after steam-curing at 70.degree. C. for 5 hours. A relatively larger amount of zeolite, up to 50% by weight of total cementing material was used in production of lightweight concrete (JP5-294749 and JP5-294750). The disclosed material comprises about 47 wt. % zeolite, 43 wt. % Portland cement, 10 wt. % quick lime and 0.07 wt. % aluminum powder. Aluminum powder, a conventional foaming agent in the production of aerated concrete, was still used in these two patent applications to provide cellular structure in the lightweight concrete. Zeolite functioned therein only as a siliceous material in the concrete composition instead of silica powder or fly ash generally used in conventional aerated concrete production.
Utilization of natural zeolite in the production of aerated concrete was reported in "Properties of zeolite as an air-entraining agent in cellular concrete", Cement, Concrete and Aggregates, CCAGDP, Vol. 14, No. 1, pp. 41-49, Naiqian Feng, 1992. The natural zeolite was calcined at about 500.degree. C. for 2 hours. The particle size of the zeolite used was less than 1.2 mm. The suggested mix proportions were 31-48 wt. % ordinary Portland cement, 19-38 wt. % zeolite and 30-38 wt. % water. The compressive strength of aerated concrete after moist-curing at 20.degree. C. for 28 days was in the range of 4.1 to 5.3 MPa and its dry density was in the range of 750 to 850 kg/m.sup.3.
Zeolite has been used as an industrial adsorbent in hazardous waste treatment. The trapped ions in some natural zeolite products include NH.sub.4, Cu, Pb, Zn, Cd, Sr and Cs. Natural zeolite has been utilized in large-scale ion exchange processes to concentrate and isolate radioactive strontium and cesium from waste streams of nuclear facilities (see, "Zeolitic extraction of cesium from aqueous solution" Unclassified Report HW-62607, US Atomic Energy Commission, 23 pp., L. L. Ames, 1960). Studies involving natural zeolites as collectors of radioactive wastes have been and are being carried out in France, Italy, Great Britain, Hungary, Bulgaria, Mexico, Canada and Japan (see "Influences of clinoptilolite on Sr-90 and Cs-137 uptakes by plants" Soil Science, Vol 114, p.149, H. Nishita and R. M. Haug, 1972). It was also reported that natural zeolite can be used in a mobile exchange unit to successfully remove 97% of ammonium from sewage streams and agricultural effluents (see, "Ammonia removal from secondary effluents by selective ion exchange", Water Pollution Control Federation Journal, Vol. 42, p. R95, B. W. Mercer, 1970). An alkali-activated-slag based binding material containing 10-30% zeolite by weight of slag has been reported for the stabilization of strontium and cesium ions (see, "Immobilization of simulated high level waste into AASC waste form" Cement and Concrete Research, Vol 24, p. 133, X. Shen et al., 1994). Low leach rates of about 10.sup.-5 and 10.sup.-6 g/cm.sup.2 .circle-solid.day for Cs and Sr ions respectively were obtained when the equivalent Cs.sub.2 O or SrO content was about 25% by weight of the binding material.
It has been reported that ettringite, hydrated calcium sulphoaluminate which has a structure Ca.sub.6 [Al(OH).sub.6 ].sub.2 (SO.sub.4).sub.3 26H.sub.2 O, can form from Al.sub.2 O.sub.3.sup.r- of the pozzolans when sulphate and calcium exist in the hydration system. The rate of ettringite formation from pozzolans is notably superior to the rate of ettringite formation from C.sub.3 A of the Portland cement (see "Ettringite from Portland cement origin and ettringite from pozzolanic origin: analogies, differences and semiquantitative relations with their respective origins: interrogations", 9th international congress on the chemistry of cement, p.343, R. Talero, 1993). Ettringite is well known both as a naturally occurring mineral and in the technology of cements as a product of the reaction of calcium sulphate with the calcium aluminates in aqueous media. Research has shown that the substitution of Al in ettringite by Ti, Cr, Mn and Fe leads to the formation of similar compounds of the type Ca.sub.6 [M(OH).sub.6 ].sub.2 (SO.sub.4).sub.3 26H.sub.2 O, which undergo extensive solid solution with each other Replacement of the SO.sub.4.sup.2- ions by CrO.sub.4.sup.2- ions is also reported to give the compound chromate-ettringite Ca.sub.6 [Al(OH).sub.6 ].sub.2 (CrO.sub.4).sub.3 26H.sub.2 O, that is isomorphous with ettringite. When calcium oxide is replaced by strontium oxide for the reaction with hydrated aluminum sulphate in water, strontium sulphoaluminate hydrate is formed which bears a close structural relationship to ettringite (see "Studies of ettringite and its derivatives", Cement Technology, Vol. 2, Part 3, p.73, 1971; "Studies of ettringite and its derivatives. Part II: Chromate substitution", Silicates Industrials, Vol 23, 1972; "Studies of ettringite and its derivatives, Part III: Investigations of strontium and barium substitution in ettringite", Cement Technology, Sept/Oct, 1972, J. Bensted and S. P. Varma). It has been concluded with respect to the utilization of cement-based solidification techniques that the fixation of metals (such as zinc and mercury) in the cementitious system is mainly attributed to substitution in the Aft phase, e.g. ettringite (see "Mechanisms of metal fixation and leaching by cement based fixation processes", Waste Management & Research, Vol. 3, p.127, C. S. Poon, A. I. Clark, C. J. Peters and R. Perry, 1985). The materials used for solidification are usually based on Portland cement and high alumina cement. Since, however, only a small part of the components, e.g. ettringite, in these materials is effective for the adsorption and fixation of toxic ions, the problems related to leakage of toxic ions from those solidified waste materials remain.
High content of zeolite in concrete compositions is economically advantageous since zeolites are abundant and relatively inexpensive. However, as stated expressly in the Japanese patent application No JP5-294750, when the zeolite content in prior art concrete compositions exceeds certain limit, the compressive strength of the resulting concrete product suffers.